It's because he's the antithesis of their team. He's humble, classy in victory and defeat, exudes a team concept, looks like a real student of the game with a passion for it and he's got a sense of humor about himself to boot.
Compare that to Brady (deadbeat dad, more concerned with his image than a Hollywood star, cocky, arrogant, a piss-poor loser, etc.) and you can see why Boston would hate him. Because deep down they wish they had him instead.